silence

first off i'd like to say that I feel dumb that it took me as long as it has to come to this conclusion. In my defense during the 2-3 months that this problem has developed i've spent a lot more time on my bikes than worrying about the car and I still don't feel badly about trying out the cheap options first etc.

anyways, symptoms-

car won't pull hard from a stop, but pulls hard from 30 mph or so on up (if i get on it already moving basically)

testing-

first thing i noticed was that fuel wasn't boosting on the launches where it didn't "go"

blower whine wasn't as "deep" on the launches it didn't "go"

At this point initially I thought it was likely a fuel delivery problem (and had plans to do some fuel upgrades soon anyways) so I replaced much of the system. This included rebuilt fmu, new fuel pump and filter, modifications to the fmu's vacuum system etc. etc., This did not fix the problem- weird!

The above steps took me about 6 weeks to complete btw because I was focused on other things. However, now this weekend is/was likely my last chance of the year to get the car out of the garage and sort out the problems. So I started messing with those other things some more looking for a solution.-

I found that the fmu boosts immediately even under very low boost when attached to a bicycle pump and manually "loaded". This means that without question my car is not creating boost on the failed launches.

so I hooked up a boost gauge and went for a drive and found that DUH! I was only boosting when i got into it when already moving...

it doesn't seem in any way logical to me that this would be a belt slip situation. I've always seen that where boost falls off then builds and falls off at higher rpms sort of thing.

It doesn't make a ton of sense to me what could be going on in the blower that would cause exactly this either. Possibly on launch is where the most stress is put on the blower and it's "slipping" internally in the snout or something? Does this make sense?

so- what's going on?

I hate these kinds of problems where I feel over my head (where the internal workings of parts I think I understand begin to baffle me)

suicidal4life

This is just a WAG but is the built in bypass in the SC working correctly? If its staying open it would hurt boost.

silence

yeah, good thinking

that is an avenue i did go down a bit in thinking/testing this

the bypass doesn't seem like it could be the problem- the vacuum-boost transition that operates it seems to be the same whether from a stop or from a roll (in all ways i can determine)

however, the MECHANISM by which it bypasses may be the culprit in some way- right? ie- the clutch or whatever it is isn't able to engage when coming from a stop. This is basically what i think is going on.

thanks

NitrogenBalance

That sucks big time. I noticed months ago that up top in the rpms I noticed there was a "wa wa wa wa" for a split second from the blower kinda like hitting a rev limiter then it was smooth again. Didn't happen often and only when the car was cold. I since replaced the HPS 90mm idler pulley that mounts on the SC bracket. I replaced it with a billet flow 100mm idler and it works perfect. No need for a longer belt either. Push down the belt a little and swap them out, 18ft lbs torqued, 5 min. Can't see that well from the picture but its a direct replacement and fixes the(my) problem.
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I'm surprised you don't have a boost gauge installed. The first thing I installed was boost/Fpressure/ and IATs. These gauges are very helpful.

silence

this isn't going to make me sound smart- but my boost gauge has been disconnected due to the remodeling of my interior. I was so convinced initially that the problem was on the fuel side I didn't even consider it...

I have boost (wasn't hooked up), fuel pressure, iat, voltage.

I think you had mentioned it before as well, but my thought was to move the pulley over "one" to the closer location when i put things back together after the winter

silence

it is the bypass not functioning properly- tested it without the bypass today and boosts fine. Sure is fun to drive too...

Just need to figure out if the bypass itself is a fault or if it's my vacuum system. seems odd it just stopped working after working for so long.

suicidal4life

Damn. Lucky guess. Good to hear you have isolated the problem. Sounds like at Higher RPM's the bypass hole is small enough it doesn't totally kill your boost like it did at lower RPM's. Hopefully it will be easy to figure out why its not functioning properly.

Edit:

Hmmm??? I wonder if there is a way we could manipulate the bypass valve and have an adjustable closing rate. Sorta like a boost controller or something like that to kill a bit of the low RPM torque and gain traction allowing you to dial in the onset of boost that the track could hold. Does that make sense or am I crazy??

silence

coult potentially be done- but unlikely to be worth the effort. also, would require more tuning capability than im running- when my car didnt boost all hell broke loose with afrs... im scaling my maf to tune and have it very dialed in as things set now, but the maf signal cant differentiate well enough btween gears to make it work as i would want it to. for what its worth he pieburg pump im now running keeps my afrssteady all the way through 4th which mu upgraded bosch could not. i would recommend this pump or larger. it also may just be butt dyno (or in this case butt gauge eading) but i feel that running 3/8s airline to my fmu and take the signal from the back of the ic core improved the speed at which my car responds to boost with more fuel. any gain there is an improvement in safety...